A conventional method for the manufacture of aluminum is by electrolysis of reduction grade Al.sub.2 O.sub.3 and utilizes a reduction pot which is lined on its side and bottom with a carbonaceous material. The reduction of Al.sub.2 O.sub.3 in a pot of this nature is carried out at temperatures of about 950.degree. to 980.degree. C. During the life of the pot, the carbonaceous material is gradually destroyed by penetration of the materials being reduced in the pot and the natural aging of the material due to its operational requirements. Finally, a pot will deteriorate to such a degree that further use of the pot is not economically feasible and must be replaced. The spent potlining may contain as much as 25 to 35 percent by weight of fluorine. Since a commercial aluminum reduction plant generates on the average of 74 pounds of spent potlining per annual ton of aluminum produced, it is clear that it is desirable to recover the fluorine in the potlining. In view of this, it is not surprising that many processes have been developed throughout the past years for recovering the fluorine from the spent potlining.
In U.S. Pat. No. 3,808,322 there is disclosed a process for making sodium fluoferrate from the sodium fluoride, NaF, obtained by processing spent potlining. In the process of this patent, spent potlining is subjected to a size reduction, calcined at 650.degree.-900.degree. C. with subsequent leaching with water, and the leachate is freed of residual impurities by adjustment of its pH to about 8.9-9.0. The remaining leachate is a substantially pure NaF solution which is then used to make sodium fluorferrate, Na.sub.3 FeF.sub.6.
Various processes for recovering the fluroine from the spent potlining using pyrohydrolysis are disclosed in U.S. Pat. Nos. 4,113,832; 4,158,701 and 4,294,816. In these processes, spent potlining and other materials, including in one instance floor sweepings, are crushed and subjected to pyrohydrolysis wherein a chemical reaction takes place. Fluoridic values such as NaF and HF can be recovered from the off gases and alumina and Na.sub.2 O values are recovered from the solid residue. In the '701 patent, the off gases are contacted in a dilute phase fluidized zone with a source of Al.sub.2 O.sub.3 to remove residual Na values and produce Na-free HF. One problem associated with the processes using pyrohydrolysis is that the reaction temperatures are such to cause the fluoride compounds to melt and coat the carbon so that it cannot burn. On subsequent solid/gaseous separation, the fluorides remain with the carbon. Additionally, rich silica levels from insulating brick, if present in the potlining, may carry over with the off gas, requiring further processing for silica removal.
An article in Light Metals, 1981, entitled "Recovery of Fluoride and Fluorine Balance of Aluminum Smelting Plant" describes the processing of spent potlining wherein the potlining is crushed, incinerated and introduced to a crusher for pulverizing and extraction of the fluorides into water. After filtration, the material is introduced into a mixing tank into which is added sulfuric acid and calcium carbonate to produce calcium fluoride. The slurry is desiccated and the sludge is sent to a disposal site. The fluorides-containing filtrate is used as the scrubbing solution in a wet-scrubber.
U.S. Pat. No. 4,113,831 discloses a process wherein the spent potlining is leached with a caustic solution followed by precipitation of sodium fluoride by saturating the leach liquor with a compound which suppresses the solubility of the sodium fluoride in the leach liquor. The preferred compound is ammonia. The precipitated sodium fluoride is removed and the leach solution saturated with ammonia is processed to remove the ammonia.
It is an object of this invention to provide a relatively inexpensive method for the recovery of fluoride values from spent aluminum potlining in a form suitable for recycling to aluminum reduction cells.
It is another object of this invention to provide a relatively inexpensive method for the recovery of fluoride values from spent aluminum potlining and any fluoride containing insulating materials associated therewith in a form suitable for recycling to aluminum reduction cells.
It is a further object of this invention to provide a relatively inexpensive method for the recovery of fluorides from spent aluminum potlining in a form suitable for recycling to aluminum reduction cells wherein the remaining residue is environmentally safe and can be disposed of as landfill material.